Of Fe-N-based compounds, .alpha."-Fe.sub.16 N.sub.2 is known as a metastable compound crystallized in the case of annealing a martensite having nitrogen dissolved therein for a long time. Though the crystal of .alpha."-Fe.sub.16 N.sub.2 is a bct structure (body-centered tetragonal system) and is expected as a giant magnetic substance having a large saturation magnetization, there has hitherto been no example to report that the compound is chemically synthesized in an isolated state.
Hitherto, to prepare .alpha."-Fe.sub.16 N.sub.2, various methods such as a vapor-deposition method, an MBE (molecular beam epitaxy) method, an ion implantation method, a sputtering method, an ammonia nitriding method, etc., have been tried. However, since .alpha."-Fe.sub.16 N.sub.2 is in a metastable phase, and .gamma.'-Fe.sub.4 N, .epsilon.-Fe.sub.2-3 N, etc., which are more stable than .alpha."-Fe.sub.16 N.sub.2, are liable to form, it is difficult to produce .alpha."-Fe.sub.16 N.sub.2 as a single compound by isolating it. Also, even when the .alpha."-Fe.sub.16 N.sub.2 crystal is obtained as a thin film, there is a limitation in the application of such a thin film to a magnetic material.
Accordingly, various attempts for producing the powders of .alpha."-Fe.sub.16 N.sub.2 have been made. For example, JP-A-8-165502 proposes a method wherein powders of Fe(N) rich in .alpha.' (martensite phase) are formed by quenching powders of Fe(N) from a .gamma. phase (austenite phase) and further grinding the powders to form a strain induced martensite, and the powders of the martensite phase are subjected to an annealing treatment to crystallize .alpha."-Fe.sub.16 N.sub.2 However, even by this method, it cannot be avoided that .alpha.-Fe remains. In fact, in the working examples described in the above-cited patent publication, the content of .alpha."-Fe.sub.16 N.sub.2 is less than 80% by weight, and thus, the product obtained by the method cannot be a powder of the .alpha."-Fe.sub.16 N.sub.2 single phase.
Also, JP-A-7-118702 proposes a method wherein a pure iron powder is reacted with a mixed gas of ammonia and hydrogen and retained at high temperatures for a long time to form a powder of an austenite single phase, the powder is quenched from an austenite single phase of a high temperature to form a powder of a martensite main phase, and the powder is then subjected to an aging treatment, whereby a powder having a trace amount of the .alpha."-Fe.sub.16 N.sub.2 phase is deposited in a martensite having nitrogen dissolved therein is formed, followed by further subjecting to a grinding treatment in a nitrogen atmosphere to accelerate the deposition of .alpha."-Fe.sub.16 N.sub.2. However, even by this method, the content of the .alpha."-Fe.sub.16 N.sub.2 phase is at most about 70% by volume.
Because in the method of crystallizing .alpha."-Fe.sub.16 N.sub.2 from a martensite phase [.alpha.'-Fe(N)], .alpha."-Fe.sub.16 N.sub.2 exists as a deposition phase in a metal texture, it is difficult to separate the crystal compound of .alpha."-Fe.sub.16 N.sub.2 as a single phase. Accordingly, by the crystallization method of .alpha."-Fe.sub.16 N.sub.2 accompanied by such phase transformation, a magnetic material of the .alpha."-Fe.sub.16 N.sub.2 single phase cannot be obtained. This is true when the material is in a powder form, and the above-described matter is also found in the method of crystallizing .alpha."-Fe.sub.16 N.sub.2 in a powder as shown in the above-cited patent publications.
Also, in the thin-film forming method, even when a thin film of .alpha.'-Fe.sub.16 N.sub.2 is obtained, use of such a thin film as a general-purpose magnetic material is problematic in productivity and economy.